There are various systems that detect the presence/absence of an object, and make a change in control status in response to the detection of the presence of an object. Assuming raindrops as an example of such an object, a windshield wiper controller for a windshield of a vehicle needs to make a change in control status thereof as circumstances demand, in response to the start of rain as a change in the weather. One of the important tasks to improve the convenience of the window wiper controller is the development of a rain sensor for detecting whether it is raining. The following will describe, as a conventional object sensor, a conventional rain sensor for detecting raindrops on a windshield of a vehicle as objects adhering thereto.
In the case of a manually-controlled window wiper that is used widely, a driver him/herself has to recognize the start of raining, consider a driving state of the vehicle and a change in an amount of raindrops adhering to a windshield, and manually turn on a window wiper switch that has been turned off, so as to secure the vision through the windshield that is indispensable while the driver drives the vehicle. To reduce the inconvenience of the manual switching operation of the window wiper, a rain sensor is provided to detect the presence of an object such as raindrops on a detection surface of the windshield of the vehicle, and determine whether the wiping of the window is necessary.
Among conventional rain sensors, a reflected-light-detecting-type rain sensor is known, which is classified as such by the raindrop detecting method. FIG. 17 is a view for simply explaining the raindrop detecting principle of a conventional reflected-light-detecting-type rain sensor. In FIG. 17, 1000 denotes a windshield of a vehicle. For conveniences in description, a space above the windshield 1000 indicates the inside of a vehicle, that is, the space on the driver side, and a space below the windshield 1000 indicates the outside of the vehicle. 1010 denotes a light source. 1020 denotes a prism for guiding light of the light source into the inside of the windshield. 1030 is a prism for guiding reflected light out of the inside of the windshield. 1040 denotes a lens. 1050 denotes a photodetecting element, and 1110 denotes a detection surface. 1120 denotes a raindrop adhering to the detection surface. Luminous flux that flares out to cover the entire detection surface is emitted from the light source 1010, and 1130 denotes a path of, among the foregoing luminous flux, a beam having been incident on a portion of the detection surface where a raindrop adheres, while 1140 denotes a path of a beam other than the beam 1130, which has been incident on a portion of the detection surface to which no raindrop adheres.
In the reflected-light-detecting-type rain sensor, it is important to select the angles (mounting angles) at which elements are mounted and properties of the materials (refractive indices of materials in particular) of the same. To describe the raindrop detecting principle briefly, light having been incident on the raindrop-adhesion portion in the detection surface leaks out to the outside since the total internal reflection condition is not satisfied on an external surface of the windshield 1000 on the outside of the vehicle, whereas light having been incident on a portion of the detection surface to which no raindrop adheres is subjected to the total internal reflection since the total internal reflection condition is satisfied on the external surface of the windshield 1000. Then, a difference between intensities of the reflected lights is determined.
Therefore, for the light source 1010 and the prism 1020, angles and materials thereof are selected so as to satisfy a condition of incidence for causing the emitted light to enter the inside of the windshield 1000, as well as the angles further being set so that the light is subjected to the total internal reflection at the detection surface in the external surface of the windshield 1000. Furthermore, an incident angle of light to the detection surface is selected so that the switching between the satisfaction and the failure of the total internal reflection condition on the detection surface 1110 can be caused according to a change in the refractive index caused by adhesion of raindrops.
For the prism 1030, an angle and a material thereof are selected so as to satisfy a condition for causing reflected light to exit the windshield 1000 to the outside, that is, so as not to satisfy the total internal reflection condition. Angles of the lens 1040 and the photodetecting element 1050 and a distance therebetween are selected so that light entering the lens 1040 is focused on a sensor portion of the photodetecting element 1050.
It should be noted that these elements 1010 to 1050 are mountable at locations other than the windshield 1000, for instance, on the hood or on the roof. However, a state to be detected is a state of the windshield 1000, and it is therefore preferable that the elements are mounted on a portion of the windshield 1000. Further, they preferably are mounted so as not to limit the driver's vision. For example, they preferably are mounted at a windshield portion behind a rear-view mirror, where the vision is already restricted because of the rear-view mirror.
To describe an operation of the foregoing conventional reflected-light-detecting-type rain sensor briefly, the luminous flux emitted from the light source 1010 is guided to the inside of the windshield 1000 by the prism 1020, so as to be incident over the entirety of the detection surface 1110. Here, it is assumed that a raindrop 1120 adheres to the detection surface 1110. Among the light incident on the detection surface 1110, the beam 1130 incident on the raindrop 1120-adhesion portion leaks out to the outside of the vehicle since the total internal reflection condition is not satisfied since the raindrop whose refractive index n is approximately 1.3 is present on the external surface of the windshield 1000. Therefore, the leaking beam is not incident on the photodetecting element 1050, and hence, it is not detected. On the other hand, among the light incident on the detection surface 1110, the beam 1140 incident on the portion without raindrops thereon is subjected to the total internal reflection since the total internal reflection condition is satisfied because of the presence of air whose refractive index n is 1 on the external surface of the windshield 1000. Then, on an internal surface of the windshield 1000 on the inside of the vehicle, the beam having been subjected to the total internal reflection is not subjected to the total internal reflection because of the presence of the prism 1030 thereon, thereby exiting toward the inside of the vehicle. The exiting beam is focused on the photosensor portion of the photodetecting element 1050 by the lens 1040.
Thus, an amount of light detected by the photodetecting element 1050 decreases because of the presence of the raindrops 1120, and the amount of the detected light decreases as an area of the detection surface 1110 covered with the raindrops 1120 increases. The change in the amount of light is detected, to detect the presence of raindrops on the detection surface 1110. This is the raindrop detecting principle of the conventional reflected-light-detecting-type rain sensor.
It should be noted that the rain sensor of the aforementioned type is configured so as to output a raindrop detection signal when it detects a change in a signal as described above. The raindrop detection signal from the rain sensor is fed to a control section of the window wiper, and in response to the input of the raindrop detection signal, a predetermined window wiper control and the like is carried out.
However, the conventional rain sensor has the following problems.
As mentioned regarding the prior art, since a raindrop detection reliability depends on a detection area, the detection surface 1110 is provided so as to have as large an area as possible to improve the raindrop detection reliability, and reflected light from the detection surface 1110 is received by a single photodetecting element 1050, so as to detect a change in the reflected light amount. For instance, from the necessity of ensuring the driver's vision through the windshield 1000 and for the purpose of not impairing the appearance of the vehicle, the rain sensor is disposed at a portion of the windshield in the vicinity of the rear-view mirror, etc., and as large an area as possible is obtained there for the detection surface use.
However, the reflected-light-detecting-type rain sensor of the prior art has a problem in its sensitivity. The conventional reflected-light-detecting-type rain sensor is required to detect a change in an amount of reflected light from the detection surface 1110 of the windshield 1000 with a high sensitivity, but the following problems make it difficult to catch a change in the reflected light amount caused by the raindrop adhesion to the windshield 1000 with a high sensitivity.
Generally, a situation in which the driver recognizes the presence of an object such as raindrops on the windshield and feels it necessary to wipe the windshield with a wiper is a situation in which raindrops in a size recognizable by the driver each adhere to the windshield. In other words, the adhesion of raindrops in the foregoing size has to be detected also on the detection surface on the windshield. In the case where a detection surface is prepared on the windshield at a position in a range that can be wiped by window wipers and in the vicinity of the rear-vision mirror, it is assumed that the presence of an object with an area ratio at a level of 0.5% to 1% has to be detected, though depending on the driver's ability of recognition. Here, taking characteristics of a photodetection signal of the reflected-light-detecting-type rain sensor of the prior art into consideration, since a light signal is detected by focusing the reflected light from the entirety of the detection surface on the photodetecting element by the lens, the reflected light amount from the entire detection surface determines a reference signal value, and only the light that exits the windshield through the raindrop-adhesion portion in the detection surface to the outside of the vehicle is involved in determining the change in the signal. Consequently, in the photodetection signal attributed to the entire detection surface, a decrease in the photodetection signal that is attributed to the raindrop-adhesion portion, for instance, a decrease of 0.5% in the photodetection signal, has to be detected. Considering that the operation is carried out in a hostile environment on the windshield of a driving vehicle with noise caused by incidence of external light and the like, normally it is extremely difficult to detect a small signal change of, for instance, 0.5%. Therefore, in the prior art, depending on a difference between an area of the detection surface 1110 and an area occupied by a raindrop to be detected, a change in the entire detection signal is likely to be buried therein.
For the above-described reason, with the conventional reflected-light-detecting-type rain sensor, it is difficult to detect a change caused by adhering raindrops in the reflected light amount with a high sensitivity.
Furthermore, regarding the conventional reflected-light-detecting-type rain sensor, there are several matters to be studied with a view to the high-sensitivity detection of a change in the reflected light amount caused by the adhesion of raindrops.
The first matter to be studied is the removal of influences of external light. The windshield 1000 is composed of a transparent substrate made of glass or the like. External light from the outside of the vehicle, including natural light such as sunlight, artificial illumination such as streetlight, neon signs, etc., and headlights of oncoming vehicles, tends to be incident on the windshield 1000, the prism 1030, the lens 1040, and the photodetecting element 1050, and is likely to be received as light noises. An amount of light that is reflected from the detection surface 1110 and focused by the lens 1040 determines a reference signal value, and a change in the signal is determined by only an amount of light that exits to the outside of the vehicle through the raindrop-adhesion portion in the detection surface 1110. In the case where an amount of the external light is significant relative to the amount of the change in the signal, it is difficult to determine whether there is a change in the detected signal fed from the photodetecting element. This leads to a problem of deterioration of the sensitivity in detecting the adhesion of raindrops.
The second matter to be studied is the selection of an angle at which light from the light source 1010 is incident on the detection surface 1110 and a refractive index of the transparent substrate so that the switching between the satisfaction and the failure of the total internal reflection condition on the detection surface 1110 can be made according to the presence/absence of an object adhering thereto. Here, an angle at which the light emitted from the light source 1010 is incident on the detection surface 1110, a property (refractive index) of the material of the windshield 1000, and a property (refractive index) of the object are to be studied mainly.
The third matter to be studied is the arrangement of elements with an angular aperture of the lens 1040 and mounting angles of the elements taken into consideration. The lens 1040 is used for receiving from the prism 1030 the light reflected by the detection surface 1110, and focusing the same on the photodetecting element 1050. The lens 1040 has an angular aperture, and this also influences the likeliness of catching external light. The angular aperture of the lens 1040 and the mounting angle of the elements have to be selected so that the lens 1040 does not catch the external light, and catches only the reflected light from the detection surface 1110. Here, a reflection angle of the reflected light from the detection surface 1110, an angle at which the light exits the windshield 1000, a property (refractive index) of a material of the prism 1030, an angular aperture of the lens 1040, a mounting angle of the lens, etc. are to be studied.
The fourth matter to be studied is the reduction of light loss in a transparent substrate such as the windshield 1000. Even a transparent substrate exhibits light loss at a ratio according to the properties of a material thereof when light is transmitted therethrough. In the case where the light loss is significant, this causes a problem of a decrease in the amount of reflected light to be received by the photodetecting element 1050.